1. Field of the Invention
The present invention relates to a switching regulator, and more particularly to a pulse modulation controlled switching regulator capable of miniaturized with high efficiency.
2. Description of the Prior Art
Conventionally, a switching regulator is indispensable as an DC-CD converter of a mobile or a portable device using a power source of communication equipment that often uses a direct current power source or a direct current power supply of dry batteries, storage batteries or the like.
In FIG. 5, there is shown an example of a general switching regulator. This switching regulator employs a pulse width modulation (PWM) controlled half bridge system in which a load voltage can be stabilized or changed by varying a pulse width, i.e., conducting period at a fixed frequency. In FIG. 5, a pulse width control circuit 21 inputs a PWM control signal to a switching elements SW1 and SW2 such as transistor, FET or the like and performs ON-OFF control of the switching elements SW1 and SW2.
When the switching element SW1 is turned ON while the switching element SW2 is OFF, a current from an input voltage Vin flows an inductor L21 on a primary side of a transformer T and a capacitor 20, excites an inductive voltage at an inductor L22 on a secondary side, is voltage-converted, and flows a rectifying and smoothing circuit 20.
In a next period, the switching element SW1 is OFF. When the switching element SW2 is turned ON, a current energy accumulated in the inductor L21 by the stored voltage of the capacitor C20 flows via this switching element SW2. The current energy accumulated in the inductor L21 is conveyed to the inductor L22 and flows the rectifying and smoothing circuit 20. The voltage and current smoothened by the rectifying and smoothing circuit 20 are outputted as an output voltage Vout.
The output voltage Vout is detected by a detecting circuit (not shown), and the detection result is fed back and inputted to the pulse modulation circuit 21. The pulse width control circuit 21 properly controls the conducting time of the switching elements SW1 and SW2, i.e., a pulse width, based on this feedback input and stabilizes the output voltage Vout. Here, a coil T is employed so that the input voltage Vin and the output voltage Vout are insulated from each other.
However, in the conventional PWM controlled half bridge switching regulator shown in FIG. 5, two switching elements are employed, and the time when a current flows while each switching element is ON or OFF and the time when a voltage is applied are partially overlapped. Therefore, there has been a problem that a switching loss occurs resulting in heat generation and noise generation from the switching elements.
Further, in the case where an ON period of the PWM control signal is short, and OFF period thereof is long, for example, in the case where a duty cycle during ON period is 10%, there has been another problem that it becomes difficult to smoothen a voltage or a current during the short ON period, a ripple increases, and an inductor and a capacitor of the rectifying circuit must be inevitably increased to decrease this ripple.